Simplified Drift Evaluation of Wall-Frame Structures

Abstract: An idealized model for analysis of multistory buildings subjected to lateral loads is presented. Using a simplified structural model composed of connecting beams and columns, along with a rapid condensation procedure, an approximated lateral stiffness matrix is obtained from which the lateral deformations can be quickly determined. The proposed model is suitable either for preliminary analysis or for quick verification of more complex computational procedures. Commonly used analytical procedures are based on one of the following two approaches: (i] rapid evaluation of story drifts based on oversimplification of structures as specified primarily by building codes (shear systemslstick models), and (ii) detailed modeling of buildings as an assemblage of structural members combined with refined analysis of each individual component (finite element analysis). While the former approach requires only a small amount of data and simple hand computations to carry out the analysis, the latter requires an abundance of information and fairly complicated and elaborate computations requiring computers. However, the first approach leads only to very approximate results often invalidated by severe assumptions and restrictions while the second method produces results which are more comprehensive and accurate. The idealization proposed in this paper is a rational combination of the two approaches leading to more accurate results with only a minimal amount of data. The simplification is obtained by assuming contraflexure points at mid spans of horizontal beams, but not extending the same simplification to vertical elements, as is typically done in simplified design and code approaches. This allows a simple representation of beams as rotational springs coupled to vertical cantilevers representing the columns. The ensuing matrix computations on a vastly reduced and diagonalized stiffness matrix can be carried out with ease on microcomputers. Using the procedure outlined herein, the internal forces in members can be reasonably obtained, and axial stresses can be roughly estimated. An application of the proposed technique is presented using two sample buildings and are compared with results of more rigorous and accurate procedures.     

Identification of Modal Combinations for Nonlinear Static Analysis of Building Structures

Abstract:   An essential requisite in performance-based seismic design is the estimation of inelastic deformation demands in structural members. An increasingly popular analytical method to establish these demand values is a "pushover" analysis in which a model of the building structure is subjected to an invariant distribution of lateral forces. Although such an approach takes into consideration the redistribution of forces following yielding of sections, it does not incorporate the effects of varying dynamic characteristics during the inelastic response. Simple modal combination schemes are investigated in this article to indirectly account for higher mode effects. Because the modes that contribute to deformations may be different from the modes that contribute to forces, it is necessary to identify unique modal combinations that provide reliable estimates of both force and deformation demands. The proposed procedure is applied to typical moment frame buildings to assess the effectiveness of the methodology. It is shown that the envelope of demands obtained from a series of nonlinear static analysis using the proposed modal-combination-based lateral load patterns results in better estimation of inter-story drift, a critical parameter in seismic evaluation and design.     

Meshfree co‐rotational formulation for two‐dimensional continua

In this paper, a meshfree co‐rotational formulation for two‐dimensional continua is proposed. In a co‐rotational formulation, the motion of a body is separated into rigid motion and strain‐producing deformation. Traditionally, this has been done in the setting of finite elements for beams and shell‐type elements. In the present work every node in a meshfree discretized domain has its own co‐rotating coordinate system. Three key ingredients are established in order to apply the co‐rotational formulation: (i) the relationship between global and local variables, (ii) the angle of rotation of a typical co‐rotating coordinate system, and (iii) a variationally consistent tangent stiffness matrix. An algorithm for the co‐rotational formulation based on load control is provided. Maximum‐entropy basis functions are used to discretize the domain and stabilized nodal integration is implemented to construct the global system of equations. Numerical examples are presented to demonstrate the validity of the meshfree co‐rotational formulation. Copyright © 2009 John Wiley & Sons, Ltd.     

Seismic Behavior of CFT Column and Steel Pile Footings

In this study, two column and pile-footing models consisting of concrete-filled steel tube (CFT) columns, reinforced concrete footings, and steel H-piles were designed and constructed at approximately one-fifth scale. The experimental differences between the two models were in the design details of the CFT column-to-footing connections. One connection consisted of welded studs, and the other used a base plate and stiffeners. The two models were tested in a vertical cantilever condition with cyclic horizontal forces and a constant axial load applied to the top of the column. The model footings were supported on 16 model steel H-piles simulating the pile foundation. Under imposed horizontal displacement, the two models with different CFT-to-pile-cap connection details demonstrated satisfactory cyclic behavior, with the development of full plastic hinges at the bottom of the columns. Strut-and-tie modeling analysis was carried out to show the force-resisting mechanism in the reinforced concrete footing. The study also validated a new design detail for the anchorage of steel H-piles to pile caps.     

对房屋抗震评估中采用的非线性静力分析方法的评估

基于设计的性能分析方法的一个关键部分是,对地震需求谱参数的准确采用。非线性静力方法(NSPs)现在广泛应用在工程实践上,主要用于预测房屋建筑的抗震能力。利用NSPs,地震需求谱可以从特定地点的危险性谱中直接计算出来,而非线性时程分析需要全部地面运动和针对地震记录可变性的相关性评估。除此之外,基于恒载模式(ATC-40和FEMA-356中所推荐的)NSP简化版本有条文说明其无法考虑由结构弹塑性而导致的更高模态效应以及模态变化。因此,研究者纷纷提出解决了以上缺点的改进Pushover法。通过对典型的钢结构和钢筋混凝土结构的突出反应特征与一套全面的NTH分析中得到的基准反应的比较,研究了多种非线性静力分析方法(NSPs)的有效性。更重要的是,考虑到多种地面运动特征,还采用了一组从普通的远断层记录到近断层地面运动的时间序列。这项分析研究的结果显示,较之该项研究中的其他非线性分析方法,适应性组合模型方法可以更为准确地预测如层间位移、塑性角等的极值。     

Effect of laminate thickness on moisture diffusion of polymer matrix composites in artificial seawater ageing

The influence of laminate thickness of polymer matrix composites on moisture diffusion in seawater immersion, as well as the resulting mechanical property degradation for composites of glass/isopolyester (G/IPE), carbon/isopolyester (C/IPE), glass/vinylester (G/VE) and carbon/vinylester (C/VE), was investigated in this paper. Laminates 3 and 10 mm in thickness, fabricated using the wet hand lay-up technique, were characterized for moisture absorption in artificial seawater medium, and their flexural strength and interlaminar shear strength (ILSS) degradations were studied. Moisture diffusion was observed to be anamolous to the Fick’s law for both 3 and 10 mm thick samples in the later stage of diffusion. Moisture permeability of 10 mm thick samples was two to three order greater than that of 3 mm thick ones, while the time to moisture saturation remained unchanged. With the increase of laminate thickness, moisture saturation increased by 1.4% for C/VE and 7% for G/IPE. The residual flexural strength and ILSS were greater in case of 10 mm thick specimens after 200 days of exposure. SEM examination of the fractured specimens showed greater levels of fibre/matrix debonding in 10 mm thick specimens.     

Discrete element method simulation of binary blend mixing of cohesive particles in a high-intensity vibration system

The effects of processing intensity, time, and particle surface energy on mixing of binary cohesive powder blends in high-intensity vibration system were investigated via discrete element method simulations. The mixedness was quantified by the coefficient of variation, C_v; lower being better. The mixing rate, which is the speed at which homogeneity was achieved, was inversely proportional to the mixing Bond number, defined as the ratio of particle cohesion to the shear force resulting from the mixing intensity. Results show that both increasing processing intensity and reducing surface energy led to a faster mixing rate. However, the mixedness improved initially as mixing action (the product of mixing rate and mixing time) increased, but later deteriorated upon its further increase. Thus, both mixing rate and mixing intensity need to be tuned for optimum mixing performance depending on the cohesion level of particles; too high or too low mixing action should be avoided.     

Response of the low-pressure hot-filament discharge plasma to a positively biased auxiliary disk electrode

In a steady-state plasma, the loss rate of plasma particles to the chamber wall and surfaces in contact with plasma is balanced by the ionization rate of background neutrals in the hot-filament discharges. The balance between the loss rate and ionization rate of plasma particles (electrons and ions) maintains quasi-neutrality of the bulk plasma. In the presence of an external perturbation, it tries to retain its quasi-neutrality condition. In this work, we studied how the properties of bulk plasma are affected by an external DC potential perturbation. An auxiliary biased metal disk electrode was used to introduce a potential perturbation to the plasma medium. A single Langmuir probe and an emissive probe, placed in the line of the discharge axis, were used for the characterization of the bulk plasma. It is observed that only positive bias to the auxiliary metal disk increases the plasma potential, electron temperature, and plasma density but these plasma parameters remain unaltered when the disk is biased with a negative potential with respect to plasma potential. The observed plasma parameters for two different-sized, positively as well as negatively biased, metal disks are compared and found inconsistent with the existing theoretical model at large positive bias voltages. The role of the primary energetic electrons population in determining the plasma parameters is discussed. The experimentally observed results are qualitatively explained on the basis of electrostatic confinement arising due to the loss of electrons to a biased metal disk electrode.     

Effect of Near-Fault Vertical Ground Motions on Seismic Response of Highway Overcrossings

Results of comprehensive nonlinear response history analyses on a range of configurations representing typical highway overcrossings subjected to combined effects of vertical and horizontal components of near-fault ground motions are reported. Current seismic design guidelines in California neglect the vertical components of ground motions for peak ground accelerations less than 0.6?g and provide rather simplistic measures to account for vertical effects when they need to be incorporated in the design. Results from the numerical simulations show that the vertical components of ground motions cause significant amplification in the axial force demand in the columns and moment demands in the girder at both the midspan and at the face of the bent cap. Axial capacity of the columns and moment capacity of the girder at the face of the bent cap were generally found to be sufficient to resist the amplification in the respective demands due to vertical effects. However, midspan moments in negative bending due to vertical motions are found to exceed the capacity of the girder. The amplified midspan moments lead to yielding of the top reinforcement resulting in average peak strains on the order of 1%. It is concluded that seismic demand analysis of ordinary highway bridges in general and overcrossings in particular should incorporate provisions for considering the adverse vertical effects of near-fault ground motions.     

An experimental research study on the effect of pictorial icons on a user-learner’s performance

This experimental counterbalanced repeated measures study on the effect of three types of icons (abstract, pictorial and drawing) on the learning and performance of two types of learners (abstract and concrete) was conducted during the Spring 2000–Spring 2001 semesters at the University of Central Florida. The counterbalanced lesson followed by a counterbalanced and completely randomized quiz on three levels was field tested in the Spring–Summer 2000 on 37 UCF graduate students and final tested in the Fall 2000–Spring 2001 on 53 UCF graduate student subjects. A general linear model repeated measures ANOVA revealed that icon type had a significant effect on the learning and performance of both types of learners. Pictorial icons resulted in best scores. No significant interaction was found between icon type and learner type even though on a matched condition, abstract learners with abstract icon resulted in better scores than when concrete learners were matched with concrete (pictorial and drawing) icons. Implications for future research are drawn as well as practical applications identified in the field of teaching, learning, training and performance.